1. Field of the Invention
The present invention generally relates to processes and systems for treating catalyst-containing substrates to recover at least some of the valuable elements from the substrates. More particularly, the present invention relates to processes and systems for treating catalyst substrates to recover catalyst promoter elements, for example, rhenium.
2. Description of Related Art
Metal catalysts are typically used to promote or enhance the reaction of many commercially significant chemical reactions. For instance, silver (Ag) impregnated on an alumina substrate serves as a catalyst for the reaction of ethylene and oxygen (referred to as an “epoxidation” reaction) to generate ethylene oxide (EO). Typically, the substrate may contain from about 8 to about 30% silver. However, the epoxidation reaction efficiency and selectivity is typically improved by the addition of catalyst “promoters” to the substrate. One of the promoters typically used in the EO epoxidation reaction is rhenium (Re).
Rhenium is a rare polyvalent transition metal and is one of the most dense elements known. Rhenium also has one of the highest melting points, and the widest range of oxidation states of any known element. Rhenium is a by-product of molybdenum refining and is distinguished as being the last naturally occurring element discovered.
Commercially, in addition to use as a promoter in EO epoxidation, rhenium is used in platinum-rhenium catalysts for the production of lead free high-octane gasoline. Rhenium is also used in high temperature alloys for jet engine parts. It is one of the ten most expensive elements on Earth.
Among other uses, there are approximately 120 EO plants in the world in the early 21st century. Each of these EO plants typically contains 80 to 500 tons of EO epoxidation catalyst, and a portion of these plants use rhenium promoted catalyst. The EO catalyst is typically replaced every one to two years as the catalytic efficiency and the selectivity of the catalyst decreases. Due to the high concentration of silver on the catalyst substrate and the high value of the silver, the silver is typically recovered and refined from the spent EO catalyst substrate.
Rhenium promoted EO catalyst substrate typically contains about 100 to about 1000 parts per million [ppm] of rhenium. The high value of rhenium and the large volume of rhenium promoted catalyst substrate used worldwide encourages the investigation of methods of recovering and refining rhenium from spent EO catalyst substrates to, for example, re-use the rhenium for new rhenium promoted catalyst substrates.
Clearly, if a process can be provided for recovering these catalyst promoters, these valuable materials can be reused and, for example, not released into the environment.